1. Technical Field
The present invention relates to an image-forming apparatus of an electrophotography system utilized in a copier, printer, facsimile machine and other such apparatus, and more particularly to a fixing device that uses heat, and a temperature control method therefor.
2. Related Art
A thermal fixing device, which heats a heater by supplying power, and uses this heat to carry out fixing as a method for fixing an electrophotographic image to a recording material, has been utilized widely for some time. In this thermal fixing device, a fixing-enabled state is achieved by supplying power to a heater, and raising the temperature until a required temperature is reached, then a recording material is conveyed to and made to pass through the fixing device while this fixing-enabled state is maintained. For this reason, as control when heating the heater, for example, the temperature of the heater is detected using a thermistor or other such temperature detecting unit, and according to the difference between this detected temperature and a preset temperature, there is 1) a phase control method, which controls the supply of power by changing the angle of energization in the alternating current half-cycle; 2) an ON/OFF control method, which turns ON the power to the heater if the detected temperature is lower than a prescribed target temperature, and turns OFF the power if the detected temperature is higher than the prescribed target temperature; and 3) a pulse-width control method, which controls the alternating current output by synchronizing the power to the heater to the zero-cross of the alternating current and changing the pulse width.
Of these methods, the ON/OFF control method generates temperature ripples due to overshoot subsequent to turning ON power, and undershoot subsequent to turning OFF power, making it difficult to achieve a stable state favorable to fixing. Using a heater having relatively high thermal capacity to reduce this overshoot and undershoot increases warm-up time, that is, the time it takes to raise the temperature of the heater to a temperature that is capable of producing a favorable fixing state. In order to achieve a device constitution that shortens warm-up time, and enables the rapid realization of a favorable fixing state, it is necessary to use a fixing device comprising a heater with low thermal capacity, or a fixing device comprising a heater with high electrical energy. However, temperature ripple is apt to occur, and reducing temperature ripple to shorten warm-up time is problematic.
Furthermore, ON/OFF control and pulse-width control that utilize a fixing device comprising a heater with low thermal capacity and high electrical energy for reducing warm-up time have the following shortcomings in addition to those described hereinabove. That is, when the heater is turned from ON to OFF, or conversely, turned from OFF to ON, flicker resulting from fluctuations in the power supply voltage occurs in the other electrical equipment, especially illumination equipment, which is connected to the same power supply line as either the fixing device or an image-forming apparatus comprising this fixing device.
By contrast, Japanese Patent No. 3,695,525, referred to herein as Patent Literature 1, discloses temperature control method for a fixing device, in which, as shown in FIG. 7, the temperature of the heater is controlled by supplying power in accordance with a first period Δt3 during which the power supply to the heater is permitted, and a second period Δt4 during which the power supply to the heater is not permitted, and in the first period, the power is supplied in a power-supply pattern comprising at least one unit of energization and at least one unit of non-energization. This temperature control method makes possible fine-tuned, high-precision temperature control that reduces power supply voltage fluctuations (ripples) in a fixing device, which comprises either a low thermal capacity or high electrical energy heating unit, or an image-forming apparatus comprising this fixing device, and, in addition, makes it possible to suppress eye-annoying flicker in other electrical equipment on the same power supply line, especially illumination equipment (a fluorescent lamp, incandescent lamp, etc.) and a display.